Radial forces of stents used in thoracic endovascular aortic repair and bare self-expanding nitinol stents measured ex vivo – Rapid rescue for obstruction of the innominate artery using bare self-expanding nitinol stents

Methods

Results

Self-expanding bare nitinol stents

Stent position

We first compared the radial forces of single bare nitinol stents between the center and edge of each stent. The radial force of the stents was significantly greater at their centers than at their edges (Table 1, F = 10.4; p = 0.0030).

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Table 1.

Radial forces of self-expanding bare nitinol stents according to the position, diameter, and type of stent graft used.

		Radial force (N)		Radial force (N)		Radial force (N)	F	p-Value
Position	Center	2.20 ± 0.05	End	1.99 ± 0.05			10.4	0.003
Diameter	8 mm	2.62 ± 0.06	10 mm	2.17 ± 0.06	12 mm	1.49 ± 0.06	101.3	<0.0001
Device	E-luminexx a	2.58 ± 0.05	Epic b	1.61 ± 0.05			222.4	<0.0001

Values presented as means ± SE. p-Values were determined by three-way analysis of variance.

a

E-luminexx® (Bard Peripheral Vascular, Tempe, AZ, USA).

b

Epic® (Boston Scientific Corporation, Cambridge, MA, USA).

Effect of stent diameter and stent type

The radial force decreased incrementally with increasing stent diameter in all stent types (Table 1, F = 101.3; p < 0.0001). We also compared the radial force between individual E-luminexx and Epic bare nitinol stents and found that the radial force was 1.6 times greater for the E-luminexx stent than the Epic stent. (Table 1, F = 222.4; p < 0.0001).

Effects of stent number

The radial force at the center of the stent was 1.9 times greater when using pairs of stents compared with single stents (Table 2, F = 71.1; p < 0.0001). The radial force decreased with increasing stent diameter for both stent types when they were subjected to compression in pairs (Table 2, F = 57.2; p < 0.0001). The radial force of two E-luminexx stents was 1.5 times greater than that of two Epic stents (Table 2, F = 169.3; p < 0.0001).

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Table 2.

Radial forces of self-expanding bare nitinol stents according to the number, diameter, and type of stent graft used.

		Radial force (N)		Radial force (N)		Radial force (N)	F	p-Value
Number	2	4.07 ± 0.06	1	2.10 ± 0.06			71.1	<0.0001
Diameter	8 mm	4.90 ± 0.11	10 mm	4.12 ± 0.11	12 mm	3.19 ± 0.11	57.2	<0.0001
Device	E-luminexx	4.92 ± 0.09	Epic	3.22 ± 0.09			169.3	<0.0001

Values presented as means of p-values were determined using three-way analysis of variance.

^aE-luminexx® (Bard Peripheral Vascular, Tempe, AZ, USA).

^bEpic® (Boston Scientific Corporation, Cambridge, MA, USA).

TEVAR endoprostheses

The radial force in response to a compression of 12 mm was significantly greater for the Valiant endoprosthesis than for the GORE c-TAG and Relay endoprostheses (Table 3, F = 29.6; p < 0.0001). Nevertheless, the radial force of the Valiant endoprosthesis compressed by 12 mm was less than the maximal radial forces of the E-luminexx and Epic stents when used singly.

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Table 3.

Comparison of the radial force of three types of endoprosthesis used in thoracic endovascular aortic repair.

		Radial force (N)		Radial force (N)		Radial force (N)	F	p-Value
Device	c-TAG a	1.14 ± 0.03	Relay b	1.22 ± 0.03	Valiant c	1.46 ± 0.03	29.6	<0.0001

Values presented as means ± SE. p-Values were determined using three-way analysis of variance.

a

GORE c-TAG® (W.L. Gore & Associates, Flagstaff, AZ, USA).

b

Relay® (Bolton Medical, Barcelona, Spain).

c

Valiant® (Medtronic Vascular, Santa Rosa, CA, USA).

Compression test of TEVAR endoprostheses and bare nitinol stents

In the compression test in which two types of stents were pressed into each other, compression of the Relay endoprosthesis by 12 mm resulted in the compression of the E-luminexx stent. In contrast, a pair of Epic stents was only slightly compressed and a pair of E-luminexx stents could not be compressed when compressing a Relay endoprosthesis by 12 mm (Figure 1).

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Figure 1.

Images of the compression tests with a Relay endoprosthesis (36 mm) and 12-mm diameter nitinol bare stents. (a) Compression of the Relay endoprosthesis and one E-luminexx without applying mechanical compression. (b) Compression of the Relay endoprosthesis by 12 mm with one E-luminexx. (c) Compression of the Relay endoprosthesis by 12 mm with two E-luminexx stent grafts. (d) Compression of the Relay endoprosthesis by 12 mm with two Epic stent grafts. The white arrow indicates the native diameter of the Relay endoprosthesis (36 mm). The yellow arrow indicates the diameter of the Relay endoprosthesis (24 mm) after being compressed by 12 mm. −, no compression; + , with compression of the Relay endoprosthesis by 12 mm.

Case report

We experienced a case in which a TEVAR endoprosthesis was used to treat a thoracic aortic arch aneurysm (Figure 2(a)). The endoprosthesis required an extended landing zone because a short proximal neck covered the innominate artery following unexpected innominate–carotid–left subclavian arterial bypass and limited blood supply to the brain. Blood flow was restored to the innominate artery by placing the middle portion of overlapped luminexx stents to the proximal edge of the aortic stent. Those luminexx stents were implanted from right brachial artery. (Figure 2(b)). At 1 year after the procedure, the patient's blood flow was intact and he had no comorbidities. Postoperative computed tomography showed that the entire circumference of the TEVAR endoprothesis was compressed by the thoracic aorta. In addition, the double-wall E-luminexx stents were compressed between the thoracic aorta and the Relay TEVAR endoprothesis in parallel (Figure 2(c) and (d)).

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Figure 2.

Images of a representative case in which the innominate artery, which supplies all of the blood to the brain, was covered by a TEVAR endoprosthesis during innominate–carotid–left subclavian arterial bypass. (a) Preoperative three-dimensional computed tomographic (3DCT) image of the aortic arch aneurysm (60-mm diameter) located just below the left carotid artery. The length of the proximal landing zone on the lesser curve of the aortic arch was 11 mm from the innominate artery. (b) Postoperative 3DCT image obtained after inserting two Relay endoprostheses (diameter × length; 42/38 × 200 mm² and 42 × 100 mm²) and two E-luminexx stents (14 × 60 mm² and 14 × 40 mm²) following innominate–left carotid–left subclavian arterial bypass. Axial (c) and cross-sectional (d) postoperative CT images show that the TEVAR endoprosthesis compressed both E-luminexx stents.

Discussion

In this study, we determined the radial forces of two types of self-expanding bare nitinol stents and three types of TEVAR endoprostheses ex vivo. Notably, we found that using two E-luminexx bare stents was appropriate for rapid rescue to restore blood flow compressed by the TEVAR endoprosthesis.

There are two methods to measure the radial force of stents. ¹⁰ The first method involves compressing the whole circumference of the stent equally. This approach provides an indication of recoil and concentric stenosis. The other method involves compressing the stent between two parallel plates and provides an indication of eccentric stenosis caused by localized smooth muscle cell proliferation. In this study, we used the second method to simultaneously compress the bare stents and TEVAR endoprostheses to mimic the clinical condition in which it is difficult to compress the entire circumferences of both stents at the same time.

The balloon-inflated stents commonly used by many centers have a greater radial force than self-expanding nitinol stents. However, balloon-inflated stents are completely inelastic, are at risk of permanent compression, and may migrate after insertion. Additionally, it takes some time to restore blood flow after inserting a balloon-inflated stent.^11–15 Considering these features, self-expanding bare nitinol stents may be more suitable to cover the innominate artery when unexpected vascular reconstruction is urgently needed.

The present study also showed that the radial force of self-expanding nitinol stents was greater at their centers than at their edges. This suggests that self-expanding stents should be placed to ensure the center of the stent is adjacent to the stenosis lesion.

We also found that the radial force decreased with increasing stent diameter, consistent with prior research. ¹⁶ Of note, the radial force of two 12-mm stents was approximately 3 N, and was three times greater than the radial force associated with compressing the circumference of the TEVAR endoprostheses to 12 mm.

The results of the compression test suggest that a pair of E-luminexx stents is resistant to compression by a TEVAR endoprosthesis. However, in the clinical case described, computed tomography demonstrated that a double E-luminexx stent was compressed by the TEVAR prosthesis. In this patient, the entire circumference of the TEVAR endoprosthesis was compressed by the thoracic aorta. This radial force was more than four times greater than the radial force measured when compressing two stents in parallel, based on a comparison between our present results and our previously reported results. ¹⁷ For this reason, the radial force placed on the TEVAR endoprosthesis increases the compression of the bare stent. Therefore, it is essential that the surgeon evaluates the possibility of stent deformities during and after surgery.

Regarding the additional stent, we hypothesized that it would be better to insert a larger diameter bare stent than multiple smaller stents because the radial force could increase with increased number of stents. In fact, the stent lumen was intact, but the stent was clearly deformed. In our previous study, the radial force associated with compressing the diameter of the stent by 12 mm was six times greater than the radial force when two stents were compressed in parallel by 12 mm. This may explain why the stent was deformed in our case. The compression ratio might be determined by the size of the TEVAR endoprosthesis relative to the aorta, the size of the stenotic lesion, and the balance between the radial force associated with compressing the TEVAR endoprosthesis and the radial force associated with compressing the bare stent in parallel.

The radial force of compressing pairs of GORE C-TAG stents was not significantly different from that of compressing the Relay and Valiant stents. However, the radial force across the entire circumference of the GORE c-TAG stent when oversizing was significantly greater than that of other two TEVAR stents. ¹⁷ Further studies are needed to clarify the radial force between the bare stent and a GORE c-TAG stent when oversizing.

Conclusion

In case a TEVAR endoprosthesis unexpectedly covered the innominate artery by implanting the proximal edge of prosthesis just below the innominate artery, which has innominate–carotid–left subclavian arterial bypass prior to placing prosthesis, placing two E-luminexx stents are effective to restore the blood flow from this coverage.